Materials Research.

2017; 20(3): 853-859 © 2017

DOI: http://dx.doi.org/10.1590/1980-5373-MR-2016-0627

The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed
Steels after Ultra Fast Cooling

Yong Tiana, Hong-tao Wanga, Yong Lia*, Zhao-dong Wanga, Guo-dong Wanga

a
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, China

Received: August 31, 2016; Revised: March 10, 2017; Accepted: April 9, 2017

In this paper, two low carbon microalloyed steels, named as steel A and steel B, were fabricated by
ultra fast cooling (UFC). In both steels, the microstructures containing quasi polygonal ferrite (QF),
acicular ferrite (AF) and granular bainite (GB) can be obtained by UFC process. The amount of AF
in steel B is more than that in steel A. The size and distribution of precipitates (Nb/Ti carbonitrides)
in steel B are finer and more dispersed than those of in steel A due to relatively low finish cooling
temperature. The mechanical properties of both steels are effectively enhanced by UFC process. UFC
process produces low-temperature transformation microstructures containing a significant amount of
AF. The mechanical properties of steel B were more satisfactory than those of steel A due to the finer
average grain size, the greater amount of the volume fractions and smaller size of secondary phases.
Keywords: Low carbon microalloyed steels, Ultra fast cooling (UFC), Acicular ferrite (AF), The
mechanical properties

1. Introduction
associated with precipitation and grain refinement10. The
High-strength low-alloy (HSLA) steels are those high- microstructure and mechanical properties of low carbon
strength structural steels having good toughness and weldability. microalloyed steel can be significantly improved by UFC
This combination of properties have led to their varied process after hot deformation11.
applications in the automotive industry, in manufacturing This article presents an analysis of the final microstructure
of large diameter pipes for gas and oil transportation in the of two low carbon microalloyed steels after UFC process.
areas of low temperature, and in fabrication of plates for Tensile and Charpy impact tests at room temperature and
naval ship’s construction1. It is also popular to use HSLA lower temperature were performed, respectively, to evaluate
steels replacing the conventional low strength counterpart strength and toughness. The key objective of this study was
for reducing thicknesses and permitting the reduction of to discuss and determine the strengthening contribution of
weight in weight-saving applications2. The wide range of the morphologies of ferrite, bainite and the detected Nb/Ti
mechanical properties attainable in HSLA steels coupled (C, N) carbonitrides in the two experimental steels subjected
with their relatively low cost are responsible for their to UFC process.
high volume of production, which represents ~10% of the
world’s steel production3. The evolution of HSLA steel 2. Experimental Procedure
was based on low carbon content to improve weldability
and suitable alloying elements were added to improve Two types of low-alloyed, low-carbon steels were produced
austenite hardenability4-6. Thermomechanical controlled in terms of different content of Cr, Mo and Ni. Plates with
processing (TMCP) and microalloying in order to obtain thickness of 250 mm were used for rolling mill tests. The
desired microstructure and properties are the essence of cylindrical rod specimens with 8 mm diameter and 15 mm
ultra-low carbon microalloyed steel7. TMCP has become length were machined from the plates in order to measure the
the most powerful and effective manufacturing process transformation temperature in a thermomechanical simulator.
to satisfy increased hardenability, improved strength, and The austenite nonrecrystallization temperature (Tnr) was
superior low-temperature toughness8. The microstructure, evaluated through softening fraction-interpass time curves.
which is related with the mechanical properties of the hot It was calculated by the back extrapolation method12. The
rolled steels, is heavily influenced by the cooling process Ar3 and Ar1, which denote the start and finish temperatures
after hot rolling. The ultra fast cooling (UFC) technology of the austenite-to-ferrite transformation, respectively,
was applied in order to get faster cooling rate. The cooling were measured using the thermomechanical simulator. A
rate of UFC is more than twice that of traditional ACC schematic illustration of the double-pass compression test
(20ºC/s for 20mm)9. UFC process enhances strengthening to measure Ar3 and Ar1 is shown in Figure 1. In Figure 1,
specimens were heated at a rate of 50ºC/s, solution treated
at 1150ºC for 180 s, deformed to 30% compressive strain at
* e-mail: neu81@126.com
854 Tian et al. Materials Research

1s-1, cooled to 920ºC at 5ºC/s, deformed to 40% compressive

strain at 1s-1. Then, the specimens were cooled at 30ºC/s to
room temperature. The chemical compositions and measured
transformation temperature (Tnr, Ar3 and Ar1) of the steels
are listed in Table 1.

Figure 2. The processing schedule of the experimental steels.

Figure 1. Schematic illustration of measuring Ar3 and Ar1. fractions of phases present in the plates were measured by
an image analyzer.
After being hold for 280 min at a soaking temperature
of 1200ºC, the rough-rolling temperature started at 1150ºC 3. Experimental Results
above the nonrecrystallization temperature of austenite for
both steels. The finish-rolling stage was started at 920ºC and 3.1. Tensile and charpy impact properties
finished rolling at 820ºC. The rolled plates were air-cooled to
the start cooling temperature of 780ºC, were water-cooled to Yield strength (Rt0.5), ultimate tensile strength (Rm), yield
550ºC and 460ºC at a cooling rate of 30ºC/s, and then were ratio (Rt0.5/Rm), total elongation (A50), the Charpy absorbed
air-cooled for steel A and B, respectively. The processing energy (AK) and Vickers hardness (HV10) of the specimens
schedule of the experimental steels is shown in Figure 2. after UFC process are summarized in Table 2. Apparently,
As a result, the final plates with thickness of 17.5 mm and steel B has higher values than steel A. The Charpy absorbed
19.3 mm for steel A and B, respectively, were obtained. energies of steel A at -10ºC are higher than 436J, and those
Five tensile and Charpy impact specimens were collected of steel B at -15ºC are higher than 396J. The hardness values
from various positions along the center-line of the plates, of steel B are also higher than those of steel A.
respectively. The flat tensile specimens, 160 mm in total length, Average ultimate tensile and yield strength, yield ratio,
20 mm in effective width, 17.5/19.3 mm in thickness and 50 total elongation and the Charpy absorbed energy of both
mm in gauge length, were machined from the plates with the steels are shown in Figure 3. In Figure 3, steel B has higher
longitudinal axis parallel to the longitudinal direction, and average value of strengths than that of steel A. Furthermore,
tensile tests were carried out on an INSTRON 4206 machine the Charpy absorbed energies level of steel B at -15ºC is
at a strain rate of 5 mm min-1. The Charpy impact specimens only slightly lower than those of steel A at -10ºC, despite
direction were also paralleled to the rolling direction. The microalloying and UFC process result in higher strength
tensile specimens were tested at room temperature, and and higher hardness for the former.
impact tests of steels A and B were performed at -10ºC
and -15ºC, respectively, according to Chinese standard to
3.2. Microstructures
obtain an averaged result13. The Vickers hardness tester was
used to measure the Vickers hardness with 10-kg load. The OM and SEM micrographs of both steels are shown in
microstructures of the transverse section of the specimens Figure 4 and Figure 5. In Figure 4, ferrite appears gray, bainite
were examined with an optical microscopy (OM) after a appears dark, and both martensite and retained austenite
LePera etching14 and with a scanning electron microscopy appear white since they are difficult to be tinted in LePera
(SEM) after conventional 4% Nital etching. Thin specimens etchant14. By visual inspection, all specimens are mostly
were observed in a transmission electron microscopy (TEM) composed of quasi polygonal ferrite (QF), acicular ferrite
with energy dispersive spectrometry (EDS) facility. Volume (AF) and granular bainite (GB). The GB contains equiaxed,

Table 1. Chemical composition (wt %) and measured transformation temperature (ºC) of both steels.
Steel C Si Mn P S Nb Cr Ni Cu Mo Ti B Tnr Ar3 Ar1
A 0.086 0.170 1.590 0.012 0.005 0.074 0.020 0.010 0.020 0.002 0.013 0.0003 950 590 480
B 0.054 0.220 1.710 0.004 0.001 0.072 0.241 0.202 0.012 0.129 0.012 0.0002 975 600 490
 The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling 855

Table 2. Mechanical properties of the experimental steels.

Specimen No. Rt0.5/MPa Rm/MPa Rt0.5/ Rm A50/% AK/J HV10
A-1 555 615 0.9 41 436 at -10ºC 219
A-2 530 615 0.86 40 408 at -10ºC 218
Steel A A-3 535 600 0.89 44 288 at -10ºC 207
A-4 515 600 0.86 40 345 at -10ºC 205
A-5 530 620 0.85 41 385 at -10ºC 220
B-1 590 675 0.87 35 394 at -15ºC 232
B-2 530 685 0.77 32 371 at -15ºC 249
Steel B B-3 535 680 0.79 34 361 at -15ºC 238
B-4 570 680 0.84 37 396 at -15ºC 243
B-5 540 680 0.79 38 331 at -15ºC 242

Figure 4. Optical micrographs of specimens after UFC process.

Figure 5. SEM micrographs of specimens after UFC process.

-15ºC for specimen B-2, are shown in Figure 6. In Figure 6,

Figure 3. Average properties of both steels.
a lot of dimples were found. The fracture appearance of steel
B still exhibits toughness characteristics even the fracture
island-shaped martensite-austenite (MA) constituents. QF and occurs at low temperature.
AF ferrite coexist in the microstructure. The QF grains have The TEM of specimens reveals the presence of quasi
irregular and jagged boundaries, containing subboundaries. polygonal ferrite (QF) and acicular ferrite (AF), martensite-
The AF is an acicular microstructure formed inside austenite austenite (MA) island, dislocation and carbide precipitation
grains and contains MA at irregularly shaped grain boundaries. at the bainitic ferrite platelet (Figures 7, 8, 9, 10 and 11).
Steel A is mostly composed of QF together with a very Specimen A-5 exhibits a mixture of QF and AF microstructure
small amount of pearlite (P). Steel B mostly consists of AF, as shown in Figure7 (a). Large amounts of AFs are found
together with GB, instead of P. in specimen B-2 as shown in Figure7 (b). The presence of
The volume fractions of secondary phases were evaluated islands of MA component within QF and/or AF structure is
and the results are shown in Table 3. Secondary phases are further confirmed by the TEM studies (Fig. 8). In Figure8 (b),
mainly retained austenite and martensite-austenite constituents, fine AFs are largely observed, while MA is homogeneously
and cementite can be ignored for low carbon content (0.086% dispersed. A fairly high dislocation density in QF and/or AF is
maximum). The volume fraction of MA tends to be higher in an essential characteristic of transformation product (Fig. 9).
steel B than that in steel A. A large number of M-A islands The precipitates with diameters in the ranges 8-40 nm and
in steel B are fine and dispersed (Figure 5, Table 3). 5-30 nm for steel A and B were observed by TEM which were
SEM micrographs of the fracture surface of the Charpy shown in Figure 10 (a) and 11 (a). The EDS image shown in
impact specimens fractured at -10ºC for specimen A-5 and at Figure 10 (b) and 11 (b) taken from the precipitate particle
856 Tian et al. Materials Research

Table 3. Average grain size and the size and the volume fractions of secondary phases.
Quasi Average size
Acicular Granular Average grain The volume fractions of The size of secondary
Steel polygonal of secondary
ferrite bainite/% size/μm secondary phases/% phases/μm
ferrite phases/μm
A Bal. - - 10.34 ± 2.3 12.67 ± 0.4 4.32~5.94 5.18 ± 0.1
B - Bal. - 9.47 ± 2.6 17.73 ± 0.5 3.54~5.12 4.36 ± 0.3

Figure 6. Fractographs of Charpy impact specimens fractured at

-10ºC and -15ºC. Figure 9. TEM micrographs of dislocations in ferrite observed
in specimens.

reveals that the particle is enriched with Nb, Ti, C and N.

The results denote the presence of carbonitrides precipitates
of niobium and titanium, Nb/Ti (C, N).

Figure 10. TEM micrograph of specimen A-5 showing the presence

of carbonitrides precipitates.

Figure 7. TEM micrographs of specimens showing the formation

of quasi polygonal ferrite (QF) and acicular ferrite (AF).

Figure 11. TEM micrograph of specimen B-2 showing the presence

of carbonitrides precipitates.

weldability. The addition of Si, Mn, Cr, Mo and other alloying

elements increase the hardenability of austenite. Austenite
has been stabilized up to a very low temperature, and γ→α
transformation has been retarded. Cr, Mo and Ni have
been added at a balanced level in steel B, which improves
Figure 8. TEM micrographs of specimens showing the presence austenite hardenability. Mo retards separation of ferrite from
of MA island. parent austenite and depresses Bs temperature. The amount
of GB in steel B is higher than that in steel A (Fig. 5). This
4. Discussion is attributed to the presence of Mo in steel B.
The soaking temperature (1200ºC) and time (280 min)
Two low-carbon steel alloyed with Si, Mn, Cr and Mo adopted in the current study essentially control the composition
and microalloyed with Nb, Ti and Ni have been designed. and grain size of austenite. The high soaking temperature
The low carbon is preferred for the current steel from the and time are employed to ensure the maximum dissolution
viewpoint of low segregation, good toughness, and superior of microalloying carbides and carbonitrides in austenite. At
 The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling 857

this high soaking temperature, austenite grain growth would are composed mainly of low-temperature transformation
be restricted by fine Nb/Ti (C, N) particles. A controlled microstructures (AF) in steel B (Table 2, Fig. 3).
precipitation process of carbides, nitrides and carbo-nitrides It was already mentioned that GB is an equiaxed
formed during the TMCP is responsible for the fine-grained microstructure, and contains island-type MA constituents. It
ferritic structure. The finish-rolling temperature is lower than is well known that MA is mainly generated at rapid cooling
the nonrecrystallization temperature (Table 1) of austenite rates and low finish cooling temperatures16. The volumetric
present in the steel and therefore static recrystallization of fraction of M-A islands increases because of higher cooling
austenite is ruled out. The pancaked grains are formed due rates20. The size of MA tended to decrease with increasing
to rolling in the nonrecrystallized austenite region. Dynamic finish cooling temperature. As hard secondary phases such as
recrystallization is likely to occur as strain accumulation MA are readily transformed at low temperatures21, therefore,
takes place from one roll pass to another in the absence of both steels show high strength due to UFC process, except
static recrystallization4. The rolling at γ nonrecrystallization for the action of Mo in the steel22. This is also the reason for
region accumulates a strain (i.e., dislocations) in austenite that the strengths of steel B were higher than those of steel
grains and this strain can promote the ferrite grain refinement A because of the strengthening contribution due to the MA
by acting as a nucleation site for γ-α transformation15. The constituent (Fig. 8).
average ferrite grain size relates to thickness of pancaked As mentioned previously, QF and/or AF contain a high
austenite grain, alloying elements that depress austenite to dislocation density (Fig. 9). This is because UFC process would
ferrite transformation and ultra fast cooling (UFC) from help to keep more dislocations introduced by deformation.
austenite region. The grain refinement effect is sufficiently The strength is mainly decided by the barriers to movement
achieved by the controlled rolling process at high temperatures, of dislocation line and deviate from M-A islands10. Therefore,
and low-temperature transformation microstructures are high strength levels were obtained for both steels, and steel
formed by the accelerated cooling process16. UFC process B ontaining a higher volume fraction and greater dispersion
is favorable to the formation of ferrite nuclei and results in of MA showed higher yield and tensile strengths.
the fine ferrite grains (Fig. 4, 5 and Table 3). The fine particle precipitation directly comes from
In general, with increased cooling rate the nature and UFC process after hot deformation for both steels. When
morphology of ferrite alters from polygonal to plate-type several of the microalloyed elements are present in the
or elongated and subsequently to lath and acicular type alloy, the precipitated carbides or carbonitrides have a
ferrite17. UFC process promotes the formation of acicular complex composition of Ti, Nb and V being more effective
ferrite (Fig. 7). According to Lu et al.18, a combination of as strengthener in the steel19,22. The suppression of grain
high cooling rate and low coiling temperature/interrupted boundary migration due to microalloying is caused by either
cooling temperature in microalloyed steels helps to produce the solute dragging effect caused by segregation of alloying
a fine bainitic/acicular ferrite microstructure, making higher elements to the boundaries, or the pinning effect caused by
strength steels possible. It is worth noting that it is easy for carbo-nitride precipitates. These very fine precipitates are
a rolling mill to produce steel products under relative higher responsible for the fine-grained ferritic structure23. Noting that
temperature rolling because the load is not heavy during the size and distribution of precipitates (Nb/Ti carbonitrides)
rolling. Therefore, the enhancement of the finishing rolling in steel B are finer and more dispersed than those of in steel
temperature (820ºC) of the present steel is favorable for A (Fig. 10, 11). Despite the precipitates reduced the absorbed
practice mill. On the other hand, a considerable amount of energy by facilitating a ductile fracture, the fine precipitation
AFs in the high start cooling temperature (780ºC) condition of the different carbides or carbonitrides (TiCN, NbCN)
is formed. It has been well accepted that the AF, in the GB during the hot-rolling provides an additional precipitation
microstructure, comes from a mixed diffusion and shear strengthening in the final steel plate.
transformation mode during continuous cooling beginning at There is some solid solution strengthening from the
a temperature above the upper bainite transformation region addition of alloying elements in two low carbon steels.
temperature19. The AF nucleates at intragranular sites and it UFC process also results in the dislocation strengthening.
is characterized by an assemblage of interwoven non-parallel Moreover, the improvement of mechanical properties also
ferrite laths with high density of tangled dislocations, where results mainly from the refinement of ferrite grain size and
the most parts of the neighboring lath (subgrain) boundaries precipitation strengthening. When the cooling rate increases,
have dissimilar orientations19. A microstructure of acicular the grain size decreases overall, and the volume fraction of
ferrite has the potential of combining high strength and high bainitic ferrite increases, which leads to the higher strength
toughness, because a crack would have to follow a more and the lower ductility and toughness16. The fine particle
tortuous path through a microstructure of acicular ferrite20. precipitation contributes to the strengthening of the steel19.
As mentioned above, the structure of steel A presents a However, the Charpy absorbed energies of steel B do not
smaller amount of acicular ferrite than steel B. The enhanced significantly reduce (Table 2 and Fig. 3). It is well known
mechanical properties are related to that the microstructures that the addition of Ni significantly increases the toughness
858 Tian et al. Materials Research

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